1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming system and an image forming method.
2. Description of Related Art
Traditionally, in an image forming apparatus which forms images while conveying a recording medium using the R2R (roll to roll) method, a continuous sheet which is the recording medium receives tension when it is being conveyed. Since the transfer device in the image forming apparatus is structured so as to sandwich an intermediate transfer belt between a secondary transfer roller and a roller facing the secondary transfer roller, the secondary transfer roller is pushed downward due to the tension applied to the continuous sheet and this may lead to the width of the secondary transfer nip which is formed by the secondary transfer roller and the roller facing the secondary transfer roller be small.
If a large pressure is applied to the secondary transfer roller to press the secondary transfer roller against the roller facing the secondary transfer roller in resistance to the tension applied to the continuous sheet, noises such as voids may occur leading to degradation in the image quality. Further, since the tension applied to the continuous sheet varies according to conditions such as glue adhesion, static and the like in the conveyance path, the width of the secondary transfer nip also varies every time the tension applied to the continuous sheet varies leading to unstable secondary transferring.
There is known a transfer device in which, in order to prevent the width of the secondary transfer nip from varying, a sheet sensor detects sheet thickness in the sheet conveyance path and a determining unit determines whether to adjust the pressing force applied to the secondary transfer roller according to the detection result obtained by the sheet sensor. By using such transfer device, the image quality can be stable even if the thicknesses of the sheets which pass through the secondary transfer nip vary (see JP 2014-089389).
Further, there is known an image forming apparatus including a detector which measures the electric resistance of a primary transfer roller and adjusts the width of the nip which is formed between the primary transfer roller and the corresponding photoreceptor drum according to the electric resistance of the primary transfer roller which is measured by the detector so that the electric resistance value of the primary transfer roller be constant at any time. In such image forming apparatus, the image quality can be prevented from being degraded due to the electric resistance of the transfer roller varying according to environmental changes and changes over time (see JP 2010-026189).
Although the device and the apparatus of JP 2014-089389 and JP 2010-026189 adjusts the nip width according to the sheet thickness and the electric resistance of the transfer roller, they are not used in the image forming where images are formed on a recording medium which is conveyed using the R2R (roll to roll) method. Therefore, there is a problem that the width at the secondary transfer nip cannot be corrected in the case where tension is applied to the continuous sheet during the secondary transferring.